This invention relates generally to fluid level sensing systems and particularly to systems in which the sensed fluid is in a vessel or tank under high pressure.
In general prior art systems sense changes in the level of a liquid in a tank or other container with a sensing element or float that is in communication with the liquid in the container and which transmits a force or movement to a control device that is situated outside the container. The force or displacement is a measure of the change in liquid level. For low pressure installations, the seal between the container and the means that transmit the force or motion from the sensor inside the container to the control means outside the container may be relatively simple. For example, a simple bellows would suffice. For high pressure environments, however, the type of seal is critical. In such an arrangement, pressures up to 6,000 lbs. per square inch (422 kilograms per square centimeter) may be encountered. Further, the means for relaying the force or motion should be capable of providing reliable and consistent operation in a variety of different environments. Also the friction imposed by the bearing and the seal should be minimal and uniform for different applications. In particular, the friction in the mechanism should be insensitive to the high pressure within the tank.
One prior art seal is shown in U.S. Pat. No. 4,700,738 in which motion is transmitted by a rotatable shaft. An 0-ring effects a seal between the shaft and the housing. The rotational type seal introduces a significant amount of friction and requires a breakout torque to begin operation which adversely affects the accuracy of the sensing mechanism. The amount of friction is also dependent upon the pressure applied to the seal.
The device illustrated in U.S. Pat. No. 4,838,303 transmits a rocking motion through the seal which flexes with movement of the transmitting shaft. The plane of the motion is defined by a single point fulcrum and a guiding mechanism. Significant friction is encountered when sliding occurs between the pin and the guiding mechanism. Also the seal may not be serviced without completely removing the sensor body from its mounting.
The trunnion bearing and seal of the present invention not only satisfies the above mentioned criteria, but has an important advantage of being field serviceable. The bearing and seal are easily removable to permit seal renewal or general maintenance. The trunnion bearing and high pressure seal of the invention also provides a replaceable low friction, high pressure liquid level sensor arrangement. The inventive apparatus consists of a pivot disk that is connected to a displacer arm (and sensor element) at one end and to a control arm (and controller) at the other end. A removable end cap carries a pair of pivot pins that are engageable with a pair of spherically shaped depressions in the face of the pivot disk. The control arm freely passes through an orifice in the end cap and is spring loaded (against the end cap) to maintain the pivot disk in engagement with the pivot pins. An elastomeric seal is provided between the end cap and a movable spacer and between the movable spacer and the pivot disk for permitting slight movement of the pivot disk about the pivot pins without disruption of the high pressure seal.